NEC - Cable Size Under Short Circuit Calculation

This section describes the fault current calculation that needs to be performed after the cable has been sized.

The adiabatic method shown below calculates maximum short circuit current. The larger the cable, the larger the current the cable can withstand without damage

Similar to the other standards it starts with source impedance from the equipment types:

• Transformer (Windings)

• Source

• Generator

For transformers, infinite primary short-circuit current is assumed to be used (known as infinite bus).

Step – I Calculate Full Load Current (IFLC)

IFLC = (Er * 1000)

V

Where:

IFLC = Full Load current in Amps

V = Phase voltage (Vph * Ö3/2) for 1 phase, Line voltage Vl for 2 phase or Line voltage (Vl * Ö3) for 3 phase

Er = Equipment rating with units in kVA

Step – II Calculate Transformer Multiplier Factor (MTR)

MTR = (100)

(Imp %)

Where:

MTR = Transformer multiplier

Imp % = Equipment Impedance expressed as a percentage

Step – III Calculate Short Circuit Current due to Source (ISC)

ISC = IFLC * MTR

Where:

ISC = Short Circuit current in Amps

IFLC = Full Load current in Amps

MTR = Transformer multiple

Steps I to III are used to calculate the short circuit due to source.

Step – IV Calculate F Factor

This section describes how the short circuit is calculated at each level rather than calculating the impedance.

This considers the calculating factor "F" of each cable for each number of phases.

• Condition: If No of Phases = 3 and cable length in feet

  f = Ö3 * ISCa * Zc

      Pn * V

Where:

L = length of conductor in feet

ISCa = available short-circuit current in amps at beginning of circuit

Zc = conductor impedance in Ohms

Pn = number of conductors in parallel

V = Line voltage

Or

• Condition: If No of Phases = 3 and cable length in Meter

  f = Ö3 * ISCa * Zc

  Pn * V

Where:

L = length of conductor in meter

ISCa = available short-circuit current in amps at beginning of circuit.

Zc = conductor impedance in ohms

Pn = number of conductors in parallel

V = Line voltage

• Condition: If No of Phases = 2 and cable length in feet

  f = 2 * ISCa * Zc

  Pn * V

Where:

L = length of conductor in feet

ISCa = available short-circuit current in amps at beginning of circuit.

Zc = conductor impedance in Ohms

Pn = number of conductors in parallel

V = Line voltage

Or

• Condition: If No of Phases = 2 and cable length in Meters

  f = 2 * ISCa * Zc

  Pn * V

Where:

L = length of conductor in meters

ISCa = available short-circuit current in amps at beginning of circuit

Zc = conductor impedance in Ohms

Pn = number of conductors in parallel

V = Line voltage

• Condition: If No of Phases = 1 and cable length in feet

  f = 2 * ISCa * Zc

  Pn * V

Where:

L = length of conductor in feet

ISCa = available short-circuit current in amps at beginning of circuit.

Zc = conductor impedance in Ohms

Pn = number of conductors in parallel

V = Line voltage

Or

• Condition: If No of Phases = 1 and cable length in Meters

  f = 2 * ISCa * Zc

  Pn * V *

Where:

L = length of conductor in meters

ISCa = available short-circuit current in amps at beginning of circuit

Zc = conductor impedance in Ohms

Pn = number of conductors in parallel

V = Line voltage

Step – V Calculate Impedance (Zc)

If impedance values (resistive and reactive components) are available from cable catalogue

Or

If NEC Table 9 values available

Or

If NEC Table 9 does not have sizes, then use this equation:

Zc = (L/1000) * Ö[(Rc)2 + (Xc)2])

Where:

Rc is resistance of active conductor (Ohms/m)

Xc is reactance of active conductor (Ohms/m)

Or

Zc = (L/1000) * Ö[(Rc)2 + (Xc)2])

Where:

Rc is resistance of active conductor (Ohms/ft)

Xc is reactance of active conductor (Ohms/ft)

Step – VI Calculate Short Circuit

ISC = ISCa * 1

(1 + f)

Where:

f = factor

ISC = Short Circuit current in Amps

ISCa = Supply short-circuit current in Amps

Step – VII Calculate cable size under Short Circuit Condition

From ICEA P-32-382, the equation to calculate the minimum conductor cross-sectional area is as follows:

• Condition - For Material = Copper

  A = Ö[((IAD/P)2 t)/(0.0297log10 [(Tf + 234) / (Ti + 234)])]

• Condition - For Material = Aluminium

  A = Ö[((IAD/P)2 t)/(0.0125log10 [(Tf + 228) / (Ti + 228)])]

Where:

IAD = short-circuit current in Amperes

t = duration of short-circuit (s)

A = conductor cross-sectional area in circular mils

Tf = final temperature (°C)

Ti = initial temperature (°C)

log10 = log 10

P = number of conductors in parallel

Note:
The option for this value is set in the project options. The options are to use Conductor Temperature for the cable or the calculated operating temperature.

The maximum short circuit temperature (Tf) values can be found:

NEC Table 8 shows the relationship between cable size in standard units and circular mil.